1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing a semiconductor device, and more specifically it relates to a semiconductor device which has a low-resistance ohmic contact that is suitable for use in high-speed device operation, and which enables the achievement and maintenance of a high-quality barrier property and high immunity to leakage current at this ohmic contact part, and to a method of manufacturing such a semiconductor device.
2. Description of the Related Art
In recent years, with ever-increasing levels of integration in semiconductor devices, there is an increasing tendency toward the short channel effect, that is, a decrease in the gate threshold voltage value, caused by the intrusion of a doped diffusion layer, which is used to separate various source and drain regions, below the gate and, to suppress this phenomenon, there is an increased need to make the junction depth of the ohmic contact part that is formed on the source and drain regions thin.
Therefore, an inevitable problem to be overcome is that of forming a contact boundary surface that is even sharper and more uniform than in the past. Metals such as titanium, cobalt, and nickel, which form suicides are coming into use for the purpose of forming self-aligning contacts.
A solid-phase siliciding reaction, however, is usually non-uniform, resulting in circular metal crystals and disturbance to the crystal structure of the diffusion layer, this leading to the tendency for leakage currents to occur.
Additionally, the silicide itself reacts easily with aluminum, the main metal that is used to form the contact wire, and particularly in the case of a non-uniform silicide film having many grain boundaries, a barrier film is necessary in order to be able to tolerate manufacturing processes at temperatures above 600xc2x0 C.
It is difficult to achieve a uniform epitaxial suicide film on a silicon (100) substrate that i~ usually used for semiconductor devices. For the case of the cobalt silicide contact of the past, as taught in the Japanese Unexamined Patent Publication (KOKAI No. 3-67334, Japanese Unexamined Patent Publication (KOKAT) No. 9-69497, Japanese Unexamined Patent Publication (KOKAI) No. 9-251967, and Japanese Unexamined Patent Publication (KOKAI) No. 10-45416, there is disclosure of forming a film (100 to 150 xcexcxcexa9-cm) from the crystal phase of high-resistance CoSi or the like, by means of a first heat treating (at around 450xc2x0 C.), after which heat treating at 600xc2x0 C. or higher is done to form a film (14 to 17 xcexcxcexa9-cm) from low-resistance crystal phase CoSi2.
However, in a method which passes through a silicide crystal phase such as that of CoSi, which has a small amount of Si, the film formed is a CoSi2 (100) film, thereby making it difficult to achieve a uniform epitaxial film.
One reason for siliciding proceeding non-uniformly is existence of various silicides that have different compositions and crystal structures.
With almost all of the metals that form silicides, at a siliciding reaction at a relatively low temperature, the heat of formation is small, and in contrast to the formation of a silicide phase having a small amount of silicon, at relatively high temperatures the heat of formation is large, this being replaced by a thermodynamically stable silicide phase having a large amount of silicon.
In the case of cobalt silicide, the participating phases are a CoSi crystal phase with a crystallization temperature of 450 to 500xc2x0 C. and a CoSi2 crystal phase with a crystallization temperature of 600xc2x0 C. or higher, and in the thermal step of the phase change from the CoSi crystal phase to the CoSi2 crystal phase, a spatially random lattice is formed, so that the final silicide thin film that is obtained is a polycrystal.
For this polycrystalline cobalt silicide, inevitable interface roughening causes increase of leakage current, worsening of thermal stability of the contact structure and increase of resistance.
Accordingly, it is an object of the present invention to improve on the above-described drawbacks in the prior art, by providing a semiconductor device wherein a ohmic contact is formed of cobalt silicide that has a high degree of surface flatness and a high uniformity, thereby enabling the achievement of a low resistance in the contact part, a low reactivity with other metals such as aluminum, and maintenance of good barrier characteristics and immunity to leakage currents, this semiconductor device featuring high-speed operation and high reliability, as well as the ability to be integrated to a high degree.
It is a further object of the present invention to provide a method for manufacturing the above-noted semiconductor device.
In order to achieve the above-noted objects, the present invention adopts the following basic technical constitution.
Specifically, the first aspect of the present invention is a semiconductor device that is provided with an ohmic contact made of a single-crystal CoSi2 film on a silicon surface (100).
The second aspect of the present invention is a method for manufacturing a semiconductor device, this method having a step of performing hydrogen termination processing of a silicon (100) surface, a step of forming a cobalt film on the silicon surface (100) that was hydrogen termination processed, a step of causing thermal diffusion of the above-noted cobalt, so as to form an amorphous cobalt silicide film on the surface part of the silicon, this having an atomic number in Co:Si ratio of 1:1.5 to 1:2.5, and a step of performing heat treating of the above-noted amorphous cobalt silicide film so as to form an ohmic contact part that is made of a single-crystal CoSi2 film.
A semiconductor device according to the present invention adopts the technical constitution described above, a feature of which is the provision of an ohmic contact part that is made of single-crystal CoSi2 film on a silicon (100) surface, the effect being a flattening on a molecular level of the surface of this ohmic contact part and the achievement of a high uniformity, this enabling the achievement of a low-resistance contact part.
In addition, there is no reactivity with other metals such as aluminum, and maintenance of good barrier characteristics and immunity to leakage currents, giving this semiconductor device improved high-speed operation, high reliability, and the ability to be integrated to a high degree.
The provision of an ohmic contact made of a single-crystal CoSi2 film on the silicon (100) surface is a new technology according to the present invention and, as discussed above, according to an invention by the inventor(s), with technology such as taught in Japanese Unexamined Patent Publication (KOKAI) No. 3-67334, Japanese Unexamined Patent Publication (KOKAL) No. 9-69497, Japanese Unexamined Patent Publication (KOKAI) No. 9-251967, and Japanese Unexamined Patent Publication (KOKAI) No. 10-45416, because the CoSi2 film that forms the ohmic contact is a polycrystal, it is not possible to achieve the special effect of the present invention.
Furthermore, in the present invention the fact that the CoSi2 film is a single-crystal film, in the usual meaning of the term, means that ideally the crystal orientation within the crystal is the same in all parts and that there are intrinsically no grain or other boundaries.
In actuality, however, if an electron beam is shone in the [100] direction using transmission electron beam diffraction, there is substantially a 99% or greater observation of diffraction spots on the CoSi2 (100) surface that corresponds with the silicon (100) surface.
In the case of a polycrystal CoSi2 film of the past, the ratio of the diffraction spots between the (100) surface and, for example, the normally observed (110) surface, is approximately 1:1, as could be calculated, for example, from the diffraction intensity.
From the electron beam diffraction image as well, it is possible to observe a polycrystal condition in which there are detectable crystal boundaries and boundary facets, for example, this enabling easy distinction of the single-crystal CoSi2 film of the present invention from the polycrystal CoSi2 film of the prior art.
In the present invention, the single-crystal CoSi2 film that forms the ohmic contact is intrinsically formed only by a cobalt disilicide (CoSi2) and intrinsically does not include other cobalt silicide phases such as cobalt silicide (CoSi) and dicobalt monosilicide (Co2Si).
This single-crystal CoSi2 film is preferably a single-crystal CoSi2 film that is epitaxially joined to the silicon (100) surface, and the phrase xe2x80x9cepitaxially joined to the silicon (100) surfacexe2x80x9d as used herein means that the crystal orientation between the CoSi2 film and the silicon simultaneously satisfies the conditions CoSi2 [100]//Si [100] and CoSi2 [011]//Si [011].
Additionally, the ideal condition for this single-crystal CoSi2 film or a single-crystal CoSi2 film that is epitaxially joined to the silicon (100) surface is one in which the thickness of the single-crystal SiCo2 film is 6 nm or less.
The reason for setting the upper limit of the thickness of the single-crystal SiCo2 film to 6 nm is that this corresponds to the upper limit of film thickness of the pre-cursor, which is the amorphous cobalt silicide, based on thermodynamics or solid diffusion theory, and it is undesirable to exceed this thickness, since exceeding this thickness would result in undesirable crystallization in the amorphous cobalt silicide. While there is no particular lower limit to the thickness of the single-crystal CoSi2 film, it is preferable in the current state of the art to make this 2 nm, in view of the need to form a uniform film.
In the present invention, after forming a single-crystal CoSi2 film that is epitaxially joined to the silicon (100) surface and which has, for example, a film thickness in the range from 2 to 6 nm, it is possible preferable from a practical standpoint to laminate onto this a single-crystal CoSi2 film or preferably a single-crystal CoSi2 film that is expitaxially joined to the above-noted CoSi2 film, this being provided as an element to form an ohmic contact.
Over the single-crystal CoSi2 that forms the ohmic contact, it is possible to laminate a layer that is of an electrode or wire material such as aluminum or tungsten.
A feature of a semiconductor device according to the present invention is the formation of a cobalt film on a hydrogen termination processed silicon (100) surface, this cobalt being then heat treated so as to form on the surface part of the silicon an amorphous suicide layer, this having an atomic number ratio in Co:Si of 1:1.5 to 1:2.5, with further heat treating being done to form an ohmic contact part made of a single-crystal CoSi2 film and preferably a (100) CoSi2 film that is epitaxially joined to the silicon (100) surface.
That is, in the present invention, hydrogen termination processing is first done to the silicon (100) surface, using for example, a fluorine solution.
This achieves hydrogen termination of, for example, dangling bonds on the silicon (100) surface. In the case of an extremely thin cobalt film, for example having a thickness of from 0.7 to 2 nm, on the silicon (100) surface, this hydrogen termination is necessary in order to achieve a uniform and continuous film.
Next, as a pre-cursor to the single-crystal CoSi2 film, instead of a cobalt silicide crystal phase such as CoSi as used in the past, an amorphous cobalt silicide layer having an atomic number ratio of 1.5 to 2.5 with respect to that of Co 1 is formed on the surface layer of the silicon.
This is based on the following knowledge of the inventors. Specifically, in general in a siliciding reaction with cobalt or the like, at the start of the reaction, at which point the treating temperature is sufficiently low, atoms of the metal cobalt, for example, diffuse into the silicon, thereby forming a mixed layer that is an amorphous silicide.
As the temperature is increased, when the phase transition temperature of the crystals near the mixture is exceeded, the crystallization of those crystal begins.
Because the composition and structure of the crystals that are formed reflect the composition of the amorphous silicide before crystallization, the silicide composition distribution after crystallization is determined by the composition distribution of the mixture layer.
In the case of cobalt silicide, to precipitate CoSi2, it is desirable to approach Co:Si composition in the amorphous silicide of 1:2.
Therefore, the idea condition for the amorphous silicide as a pre-cursor is a composition of 1.5 to 2.5 silicon with respect to 1 cobalt.
If the amount of silicon is less than 1.5 or greater than 2.5 with respect to Co 1, the final CoSi2 film that results is not a single crystal, and when observed with an electron microscope from the side, clearly loses its flatness, and it is possible to observe within the crystal steps, for example a (111) surface facet that is formed.
Upon further investigation and study based on the above-described conceptual idea, the inventors were able to arrive at the discovery that, by forming the above-described amorphous cobalt silicide as a pre-cursor, it is possible to achieve an unexpected film made of single-crystal CoSi2 and also a single-crystal CoSi2 that is epitaxially joined to the silicon (100) surface, this ultimately leading to present invention.
Therefore, it should be noted, the preferable ideal condition for forming a single-crystal CoSi2 film or a single-crystal CoSi2 that is epitaxially joined to the silicon (100) surface, in order to limit the amount of cobalt and make the amount of silicon large, is to have the cobalt film thickness be 2 nm or thinner, and also to have the thickness of the CoSi2 film be 6 nm or thinner.
The reason for setting the upper limit of the cobalt film thickness to 2 nm and the upper limit to the CoSi2 film thickness to 6 nm is that the upper limit of film thickness of the amorphous cobalt silicide, based on thermodynamics or solid diffusion theory, corresponds to the upper thickness limits of the cobalt film and CoSi2 film, and if the cobalt film thickness exceeds 2 nm, there is a tendency toward undesirable crystallization in the amorphous cobalt silicide.
While there is no particular lower limit to the thickness of the cobalt film and the single-crystal CoSi2 film, from the standpoint of forming a uniform film, it is usual in the current state of the art to make the lower limit of thickness of the cobalt film 0.7 nm and the lower limit of thickness of the single-crystal CoSi2 film 2 nm.
In connection with the above, it is preferable in practicing the present invention to form, for example, a single-crystal (100) CoSi2 film having a thickness in the range from 2 to 6 nm that is epitaxially joined to the silicon (100) surface, and then laminated onto this as an element to form an ohmic contact a single-crystal CoSi2 film having an arbitrary thickness, this preferably being a single-crystal (100) CoSi2 film that is epitaxially joined to the (100) CoSi2 film.
Next, it is preferable to perform thermal diffusion of the cobalt while maintaining a temperature that is below the crystal phase transition temperature of cobalt silicide, this being the range from 300xc2x0 C. to 450xc2x0 C. The is because at a temperature below 300xc2x0 C., the diffusion of cobalt requires an excessive amount of time, one hour or more being required for heat treating.
On the other hand, at a temperature that exceeds 450xc2x0 C., before sufficient diffusion is reached, precipitation of cobalt silicide crystals other than CoSi2, such as CoSi will begin.
The heat treating of the amorphous suicide is preferably performed at a temperature of 600xc2x0 C. or greater, which is suitable for the generation of CoSi2, this temperature range making it possible for an epitaxial relationship to be formed between the CoSi2 and the silicon.
For example, if a uniform silicide film having a CoSi2(100)/Si(100) epitaxial boundary is first obtained, the structure is stable, after which this structure is maintained, even with heat treating at a temperature exceeding 800xc2x0 C.
If an extremely uniform epitaxial film such as this is achieved, it is possible to suppress mutual diffusion with respect to electrodes and wiring made of, for example, aluminum on the ohmic contact part.
An experimental comparison of the physical properties obtained upon practicing the method of manufacturing a semiconductor device according to the present invention revealed the following results.
Specifically, a Si (100) substrate was treated with BHF (buffered hydrofluoric acid), thereby removing the natural oxide film from the surface and performing hydrogen termination of the surface.
Next, the substrate was transported into a high vacuum, where an electron beam gun was used to deposit cobalt to a thickness of 1 nm, while maintaining the chamber temperature.
Then, the substrate temperature is maintained at 400xc2x0 C. for ten minutes, after which it is slowed raised (at a rate of 20xc2x0 C./minute) until it reaches 700xc2x0 C., immediately after which the temperature is dropped.
If the structure of this Si (100) surface is observed using reflection high-energy electron diffraction (RHEED), a spot diffraction pattern corresponding to polycrystal cobalt after deposition of the cobalt appears, and at heat treating at 400xc2x0 C., there is a gradual change to a hollow pattern that corresponds to amorphous cobalt silicide.
If the temperature is further increased to 700xc2x0 C., the pattern changes to a streak pattern that indicates epitaxial silicide which completely reflects the substrate orientation.
If the cross-section of the film thus obtained is observed using a high-resolution transmission electron microscope (TEM), it is possible to observe the alignment of the silicon crystal lattice of the substrate, as well as a CoSi2 crystal layer that is arranged in the (100) direction.
The boundary is flat to within a level of 1 or 2 atoms, and the film thickness is nearly uniform at approximately 3 nm.
In contrast to the above results, in the case in which a cobalt film having a thickness of 2 nm is heat treated with a final temperature of 600xc2x0 C. without performing hydrogen termination, electron diffraction of the CoSi film that is formed revealed a polycrystal condition having a diffraction intensity ratio between diffraction lines in the (100) and (110) directions of 60:40, this film being not continuous but rather exhibiting a separated island shape.
If, for example, a hydrochloric acid Branson cleaning solution is used as the final cleaning solution instead of the original BHF, a thin natural oxide film will remain on the surface, the final thickness of the film that is formed being non-uniform and not an epitaxial film.
That is, in a method for manufacturing a semiconductor device according to the present invention, it is necessary to go through the processes of hydrogen termination and the formation of an amorphous cobalt silicide phase having a specific composition, and if either of these conditions is not satisfied, it is not possible to achieve a cobalt silicide, that is, a single-crystal CoSi2 film and preferably a single-crystal (100) CoSi2 film that is epitaxially joined to the silicon (100) surface which is suitable for the formation of an ohmic contact.